Method for charging batteries

ABSTRACT

A method for charging and monitoring a rechargeable battery is proposed. The charging method includes charging a rechargeable battery, sensing temperature of said battery, and calculating temperature change rate. The method further includes sensing voltage of said battery after the temperature change rate reaches a predetermined value, calculating voltage change rate, and reducing current sent to the battery when the voltage change rate reaches a predetermined value. A second embodiment of the charging method may include charging a rechargeable battery, sensing temperature and voltage of said battery, and calculating temperature and voltage change rates. The method further includes sensing for a maximum voltage change rate after the temperature change rate reaches a predetermined value, and reducing current sent to the battery when the maximum voltage change rate is sensed. In addition, the method may include sensing for a minimum voltage change rate before sensing for the maximum change rate, wherein sensing for the maximum change rate would begin after the temperature change rate reaches a predetermined value or after the minimum voltage change rate is sensed.

This application claims benefit of Provisional Application Ser. No.60/056,758 filed Aug. 20, 1997.

FIELD OF THE INVENTION

This invention relates generally to a method for charging and monitoringrechargeable batteries.

BACKGROUND OF THE INVENTION

The several advantages of cordless power for portable power tools andcertain kitchen and domestic appliances have led to the development of awide range of sizes of power- or battery-packs, that is, a containedgroup of power cells. These power cells may include nickel cadmium(NiCd), nickel metal hydride (NiMH), lithium, or lead-acid cells, etc.

FIG. 2 shows illustrating a typical voltage and temperature curves of abattery during charging. As shown in the voltage curve V, the voltage ofthe battery typically does not increase linearly. Instead, the batteryvoltage increases until an area A is reached, where the battery voltageincreases slowly. The battery voltage then rapidly increases until avoltage peak B is reached. Full charge of the battery occurs just beforevoltage peak B.

If the charging process is not terminated, the battery would then beovercharged, possibly damaging the battery. As shown in the voltagecurve V, the battery voltage also decreases when the battery isovercharged.

During the charging process, the battery temperature also varies. FIG. 2shows a typical temperature curve T for a battery being charged, whenthe battery temperature before charging is around room temperature. Asindicated in temperature curve T, the battery temperature begins to risearound or after the area A is reached. The temperature then continues toincrease until the charging process is terminated. If the chargingprocess is not stopped when the battery is fully charged, the batterycould be overcharged and thus damaged by the rising temperature.Accordingly, battery temperature or battery voltage are usuallymonitored as indicators of the full charge condition.

Among the voltage monitoring methods, the Saar double inflectiontermination method described in U.S. Pat. Nos. 4,388,582 and 4,392,101,is preferred to detect a battery reaching full charge. However, whilethe double inflection method avoids overcharging of a battery that wascompletely discharged, the method is difficult to implement with alreadyfully or mostly charged batteries, as well non-expressive batteries,such as NiMH batteries, which have depressed voltage curves.

Other voltage monitoring methods more typically employed are (1) theminus-delta-voltage method, (2) the peak detect method, and (3) thevoltage slope detect method. In the minus-delta-voltage method, a sampleof the battery peak voltage is stored and compared to the most recentvoltage. Termination occurs when the most recent voltage falls below aset point, usually within between 0.5% and 1.0% of the stored peak, orabout 10 to 20 millivolts per cell.

The peak detect method is more modem version of the minus-delta-voltagemethod. Basically, the same method is used, except the set point can beset closer to the peak by using more accurate instrumentation. Both ofthese methods, however, tend to overcharge the battery, reducing batterylife.

The slope detect method is another voltage monitoring method. Accordingto this method, the voltage peak B is detected by calculating the slopeof the voltage curve V, or voltage change rate (dV/dt). Terminationoccurs when the voltage change rate is 0 or negative. This method alsotends to overcharge the battery, reducing battery life.

However, the slope detect method has another drawback. Under thismethod, the area A may be confused with voltage peak B because of thesmall slope. This could cause the termination of the charging process,resulting in undercharged batteries.

Current temperature monitoring methods are also problematic. Temperaturemonitoring methods typically employed are (1) absolute temperaturetermination and (2) temperature change rate termination. Absolutetemperature termination relies on the temperature rise that occurs whenthe battery is fully charged. Under this method, the charging processwill be stopped when the battery temperature reaches a certaintemperature. However, because the largest temperature rise usuallyoccurs after the battery is frilly charged and during the overchargeperiod, battery life and performance are adversely affected.

The temperature change rate termination method requires monitoring thechanging slope of the battery temperature, or temperature change rate(dT/dt), during the charging process. Termination occurs when thetemperature change rate reaches and/or exceeds a predetermined rate. Inother words, termination occurs when a trip point is reached and/orexceeded. However, selecting the appropriate trip point is problematic,especially under conditions of varying ambient temperature conditions.Accordingly, the method may cause undercharged or overcharged batteries.

It is preferable to provide a charging and monitoring method that willnot result in undercharged or overcharged batteries.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method for charging andmonitoring a rechargeable battery is proposed. The charging methodincludes charging a rechargeable battery, sensing temperature of saidbattery, and calculating temperature change rate. The method furtherincludes sensing voltage of said battery after the temperature changerate reaches a predetermined value, calculating voltage change rate, andreducing current sent to the battery when the voltage change ratereaches a predetermined value.

Similarly, the charging method may include charging a rechargeablebattery, sensing temperature and voltage of said battery, andcalculating temperature and voltage change rates. The method furtherincludes sensing for a maximum voltage change rate after the temperaturechange rate reaches a predetermined value, and reducing current sent tothe battery when the maximum voltage change rate is sensed. In addition,the method may include sensing for a minimum voltage change rate beforesensing for the maximum change rate, wherein sensing for the maximumchange rate would begin after the temperature change rate reaches apredetermined value or after the minimum voltage change rate is sensed.

Additional features and benefits of the present invention are described,and will be apparent from, the accompanying drawings and the detaileddescription below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of theinvention according to the practical application of the principlesthereof, and in which:

FIG. 1 is a circuit schematic diagram of a battery to be chargedaccording to the method of the present invention;

FIG. 2 is a graph illustrating the voltage and temperature curves forthe battery of FIG. 1 during the charging period;

FIG. 3 is a flowchart illustrating the monitoring and charging processaccording to the present invention; and

FIG. 4 is another flowchart illustrating an alternative monitoring andcharging process according to the present invention.

DETAILED DESCRIPTION

The invention is now described with reference to the accompanyingfigures, wherein like numerals designate like parts. The apparatusdisclosed below, however, is only disclosed for exemplary purposes, asthe proposed method can be carried out by other battery-chargercombinations. In addition, all the teachings of the Saar U.S. Pat. Nos.4,388,582 and 4,392,101 are hereby incorporated by reference into thisspecification. Referring to FIG. 1, a battery 10 is connected to acharger 20. Battery 10 comprises one or more battery cells 11, whichdictate the voltage and storage capacity for battery 10.

Battery 10 includes three battery charging contacts: first batterycontact 12, second battery contact 13, and third battery contact 14.Battery contact 12 is the B+ (positive) terminal for battery 10. Batterycontact 13 is the B- or ground terminal. Battery contact 14 is the S orsensing terminal.

As shown in FIG. 1, the battery cells 11 are coupled between the batterycontacts 12 and 13. In addition, coupled between battery contacts 13 and14 is a temperature sensing device, such as a negative temperatureco-efficient NTC) resistor, or thermistor, R_(T). The temperaturesensing device allows monitoring of the battery temperature. Personsskilled in the art will recognize that other components, such ascapacitors, etc., or circuits can be used to provide a signalrepresentative of the battery temperature.

The charger 20 includes positive and negative (B+ and B-) terminalswhich are coupled to battery 10 via battery contacts 12 and 13. Thepositive terminal may also act as an analog/digital input A/DINPUT_(V),in order to detect the battery voltage. In addition, the charger 20includes an analog/digital input A/DINPUT_(T), which is coupled tobattery 10 via the third battery contact 14 (S). This allows the charger20 to monitor the battery temperature.

FIG. 3 is a flowchart of the different steps included in the proposedmethod. The first step (ST1) is to begin the charging process. Thecharger 20 sends a current into the battery, in order to recharge thebattery. The charger 20, via its A/D inputs, obtains the original, orstarting, battery temperature T0 (ST2). The charger 20 continues tomonitor and store the battery temperature (ST3) in order to calculatethe temperature change rate (dT/dt) (ST4).

The charger 20 then checks whether the temperature change rate is equalto or exceeds a predetermined temperature change rate X (ST5). Thechange rate X is usually empirically selected to ensure that the batteryvoltage will be between the area A and the voltage peak B when thechange rate X is reached, as shown in FIG. 2. For example, anappropriate change rate trigger for many batteries would between 0.1 and2 degrees per minute, with a range between 0.5 and 1.2 degree per minutebeing the preferred range for NiCd batteries. If the predeterminedchange rate X has not been reached, the charger 20 sets the originaltemperature T0 to be the latest sensed temperature T (ST6), senses thenew battery temperature T (ST3) and calculates the temperature changerate (ST4). The charger 20 will continue this process until thepredetermined change rate X is reached.

Once the predetermined change rate X is reached, the charger 20 thensenses the original, or starting, battery voltage V0 (ST7). The charger20 continues to monitor and store the battery voltage (ST8) in order tocalculate the voltage change rate (dV/dt) (ST9). The charger 20 thenchecks whether the voltage change rate is equal to or less than apredetermined voltage change rate Y (ST10). For example, thepredetermined voltage change rate may be about zero volts per minute sothat the voltage peak B is identified. However, persons skilled in theart will recognize that selecting other change rates, such as smallpositive change rates, will allow termination of the charging processbefore the voltage peak B is reached, assuring that the battery will notbe overcharged. Accordingly, the predetermined voltage change rate Ypreferably is less than 0.2 millivolts per cell per minute andpreferably within 0.1 and 0.0 millivolts per cell per minute.

Once the predetermined voltage change rate Y is reached, the chargingprocess is slowed by reducing the current being sent to the battery(ST12). It is preferable to reduce the current so that the batteryreceives a maintenance charge or a reduced charge. (See Saar U.S. Pat.Nos. 4,388,582 and 4,392,101). The maintenance charge current can thenbe completely cut off after a certain period of time. Nevertheless,persons skilled in the art will recognize that, if current is not sentto the battery once the predetermined voltage change rate Y is reached,the current has in effect been reduced.

If the predetermined change rate Y has not been reached, the charger 20sets the original voltage V0 to be the latest sensed voltage V (ST11),senses the new battery voltage V (ST8) and calculating the voltagechange rate (ST9). The charger 20 will continue this process until thepredetermined change rate Y is reached.

Persons skilled in the art will recognize that the process describedabove can be done in parallel, i.e., the charger 20 will conduct thevoltage and temperature comparisons at the same time rather thansequentially.

FIG. 4 is a flowchart of the different steps included in an alternativeembodiment of the proposed method. The first step (ST20) is to begin thecharging process. As discussed above, the charger 20 sends a currentinto the battery, in order to recharge the battery. The charger 20obtains the original, or starting, battery temperature T0 and original,or starting, voltage V0 (ST21). The charger 20 then sets original, orstarting, voltage change rate dV/dt(0) equal a value M. The value M maybe a high value and is preferably a practically unrealistic high value.

The charger 20 continues to monitor and store the battery temperatureand voltage (ST23) in order to calculate the temperature change rate(dT/dt) (ST24). The charger 20 then checks whether the temperaturechange rate is equal to or exceeds a predetermined temperature changerate X (ST25). As discussed above, the temperature change rate X isusually empirically selected to ensure that the battery voltage will bebetween the area A and the voltage peak B when the change rate X isreached, as shown in FIG. 2. For example, an appropriate change ratetrigger for many batteries would between 0.1 and 2 degrees per minute,with a range between 0.5 and 1 degree per minute being the preferredrange for NiCd batteries.

If the predetermined temperature change rate X has not been reached, thecharger 20 calculates the voltage change rate (dV/dt) (ST26). Thecharger 20 checks whether the voltage change rate is equal to or smallerthan the original voltage change rate (ST27). If so, the charger 20 setsthe original voltage change rate to equal the voltage change rate(ST28). By doing so, the charger 20 can later detect when the latestvoltage change rate is larger than the original voltage change rate, andthus identify whether a minimum change rate has been reached (whichwould be near point L in FIG. 2). The charger 20 then continues to sensethe battery temperature and voltage (ST23) and checking whether thepredetermined temperature change rate X has been reached.

If the predetermined temperature change rate X has been reached, or ifthe minimum voltage change rate has been reached, the charger 20 thenbegins sensing for the maximum voltage change rate. To do so, thecharger 20 sets the original voltage change rate to equal the latestvoltage change rate (ST29). The charger 20 then senses the currentbattery voltage (ST30) and calculates the voltage change rate (ST31).

The charger 20 checks whether the voltage change rate is equal to orlarger than the original voltage change rate (ST32). If so, the chargercontinues to set the original voltage change rate to equal the latestvoltage change rate (ST29), sense the battery voltage (ST30) andcomparing the voltage change rates (ST31 and ST32), until the latestvoltage change rate is smaller than the original change rate. When thiscondition is met, the maximum voltage change rate has been sensed andidentified (usually near point H in FIG. 2), signaling that the voltagepeak B is relatively near.

In order to prevent overcharging of the battery, it is preferable slowthe charging process by reducing the current being sent to the battery(ST33). Again, it is preferable to reduce the current so that thebattery receives a maintenance charge. (See Saar U.S. Pat. Nos.4,388,582 and 4,392,101). The maintenance charge current can then becompletely cut off after a certain period of time. Nevertheless, personsskilled in the art will recognize that, if current is not sent to thebattery once the maximum voltage change rate has been sensed, thecurrent has in effect been reduced.

Persons skilled in the art may recognize other alternatives or additionsto the means or steps disclosed herein. For example, level thresholdsmay be implemented in the monitoring and charging method in order toavoid proceeding to the next step based on signal noise,nonrepresentative measurements, etc. However, all these additions and/oralterations are considered to be equivalents of the present invention.

What is claimed is:
 1. A method for charging rechargeable batteriescomprising the steps of:charging a rechargeable battery; sensingtemperature of said battery; calculating temperature change rate;sensing voltage of said battery after the temperature change ratereaches a first predetermined value; calculating voltage change rate;and reducing current sent to the battery when the voltage change ratereaches a second predetermined value.
 2. The method of claim 1, whereinthe current reducing step comprises not sending any current to thebattery.
 3. The method of claim 1, wherein the first predetermined valueis between about 0.1 and 2 degrees per minute.
 4. The method of claim 1,wherein the first predetermined value is between 0.5 and 1 degrees perminute.
 5. The method of claim 1, wherein the second predetermined valueis less than 0.2 millivolts per cell.
 6. The method of claim 1, whereinthe second predetermined value is less than 0.1 millivolts per cell. 7.A method for charging rechargeable batteries comprising the stepsof:charging a rechargeable battery; sensing temperature and voltage ofsaid battery; calculating temperature and voltage change rates; sensingfor a maximum voltage change rate after the temperature change ratereaches a first predetermined value; and reducing current sent to thebattery when the maximum voltage change rate reaches a secondpredetermined value.
 8. The method of claim 7, further comprising thestep of sensing for a minimum voltage change rate before sensing for themaximum change rate.
 9. The method of claim 8, wherein sensing for themaximum change rate begins after the temperature change rate reaches thefirst predetermined value or after the minimum voltage change ratereaches a third predetermined value.
 10. The method of claim 7, whereinthe current reducing step comprises not sending any current to thebattery.
 11. The method of claim 7, wherein the first predeterminedvalue is between about 0.1 and 2 degrees per minute.
 12. The method ofclaim 7, wherein the first predetermined value is between 0.5 and 1degrees per minute.
 13. The method of claim 7, wherein the secondpredetermined value is less than 0.2 millivolts per cell.
 14. The methodof claim 7, wherein the second predetermined value is less than 0.1millivolts per cell.
 15. A charger for charging rechargeable batteriescomprising:at least three terminals for connecting the charger to arechargeable battery; a current source connected to at least one of theterminals for providing current to the battery; a controller connectedto the current source and at least one of the terminals for controllingthe current sent to the battery, sensing battery voltage and temperatureand calculating temperature and voltage change rates; wherein saidcontroller senses voltage of said battery after the temperature changerate reaches a first predetermined value, and reduces current sent tothe battery when the voltage change rate reaches a second predeterminedvalue.
 16. The charger of claim 15, wherein the first predeterminedvalue is between about 0.1 and 2 degrees per minute.
 17. The charger ofclaim 15, wherein the second predetermined value is less than 0.2millivolts per cell.
 18. A charger for charging rechargeable batteriescomprising:at least three terminals for connecting the charger to arechargeable battery; a current source connected to at least one of theterminals for providing current to the battery; a controller connectedto the current source and at least one of the terminals for controllingthe current sent to the battery, sensing battery voltage and temperatureand calculating temperature and voltage change rates; wherein saidcontroller senses for a maximum voltage change rate after thetemperature change rate reaches a first predetermined value, and reducescurrent sent to the battery when the maximum voltage change rate reachesa second predetermined value.
 19. The charger of claim 18, wherein thecontroller senses for a minimum voltage change rate before sensing forthe maximum change rate.
 20. The charger of claim 19, wherein sensingfor the maximum change rate begins after the temperature change ratereaches the first predetermined value or after the minimum voltagechange rate reaches a third predetermined value.